1. Field of the Invention
The present invention relates to a golf ball and, more particularly, to an improvement of the aerodynamic characteristics of the outer surface of a golf ball having a plurality of dimples formed in the outer surface of the ball.
2. Disclosure of Related Art
Generally, a golf ball flying in the air is subject to two types of air resistance, i.e., a pressure drag produced by an air pressure difference produced in front of and behind the ball, and a friction drag produced by friction between the surface of the ball and the air. These resistances decrease the distance of flight of the ball. Generally, the friction drag imposed on the ball during flight is much less than the pressure drag imposed on the ball during flight. Therefore, to increase the distance of flight of the ball, it is desirable to reduce the pressure drag imposed on the ball, as much as possible.
When a ball is flying in the air, a laminar air flow boundary layer is produced on the front side of the ball, and a turbulent air flow boundary layer, i.e., an intermediate boundary layer at the point of transition from a laminar flow to a turbulent flow, is produced on the outer surface of the ball behind the laminar air flow boundary layer, and at the rear end of the turbulent air flow boundary layer, the air stream is exfoliated from the outer surface of the ball and a plurality of swirling streams are produced behind the ball. In front of the ball, air is pressurized by the ball, and thus the air pressure is increased, but behind the ball, the air pressure is reduced due to the exfoliation of the air stream from the outer surface of the ball. Therefore, a pressure difference occurs in front of and behind the ball during the flight, and accordingly, a resistance force is produced by this pressure difference which acts on the ball to disturb the advance of the ball. Such a resistance force is known as pressure drag. It is known that the magnitude of the pressure drag imposed on the ball during flight is roughly in proportion to the square of a kinetic speed of the ball.
In order to reduce the pressure drag imposed on the golf ball during the flight, it is desirable to reduce the air pressure difference produced in front of and behind the ball as soon as possible by reducing the pressure drop produced behind the ball. Accordingly, it is desirable to improve the aerodynamic characteristics of the outer surface of the ball so that the turbulent air flow boundary layer extends toward the front and back of the ball, and the exfoliating point of the air stream shifts toward the back of the ball.
In conventional golf balls, a spherical body is formed with a plurality of circular dimples uniformly arranged in the entire outer surface thereof. These dimples serve to shift the exfoliating point of the air stream toward the back of the ball.
When the relationship between the kinetic speed of the golf ball and the air pressure drag imposed on the ball during flight is examined, the air pressure drag imposed on the ball increases gradually in accordance with the increase of the ball speed, but immediately after the ball speed exceeds a certain speed, i.e., a critical speed, the air pressure drag is abruptly reduced, and then the pressure drag also increases gradually in accordance with the increase of the ball speed. Such an abrupt decrease of the pressure drag occurs due to the production of a turbulent air flow boundary layer on the outer surface of the ball. Generally, the kinetic speed of a golf ball hit by a golf club is in a range of 20 to 70 m/sec: an initial speed of the ball being in a range of 40 to 70 m/sec; and the speed of the ball during falling being in a range of 20 to 30 m/sec. When a golf ball having a smooth spherical outer surface without dimples is hit by a golf club, an abrupt drop of a pressure drag due to a production of a turbulent air flow boundary layer occurs at a speed of about 60 m/sec. That is, a critical speed of a ball having a smooth outer surface is about 60 m/sec. Therefore, such a ball having a smooth outer surface is subject to a small pressure drag during flight at a high speed of 60 to 70 m/sec, but is subject to a greater pressure drag during flight at a low and medium speed of 20 to 60 m/sec, resulting in a decrease of the distance of flight of the ball and a deterioration of directional control of the flight of the ball. In the case of a conventional golf ball having dimples on the outer surface thereof, a critical speed is about 25 to 30 m/sec. Therefore, such a ball having dimples is subject to a small pressure drag during flight at the medium and high speeds, but is subject to a greater pressure drag during flight at the low speed, particularly when the ball falls, resulting in a decrease of the distance of flight of the ball.